1. Field of the Invention
The present invention relates to a sheet feeding apparatus adapted to feed a sheet to an image forming portion or an image reading portion of an image processing system such as a copying machine, facsimile, printer and the like.
2. Related Background Art
In conventional sheet feeding apparatuses, as a driving source for shifting a pick-up roller toward and away from a sheet, a solenoid was used (as disclosed in the Japanese Patent Laid-Open No. 48-96023) or a cam was used (as disclosed in the Japanese Patent Publication No. 62-19330).
However, in the conventional sheet feeding apparatus disclosed in the above Japanese Patent Laid-Open No. 48-96023, another solenoid for separating the pick-up roller from the sheet is provided, independently of the driving source for rotating the pick-up roller. On the other hand, in the conventional sheet feeding apparatus disclosed in the above Japanese Patent Publication No. 62-19330, clutches are arranged between the driving source and the pick-up roller and between the driving source and the cam for separating the pick-up roller from the sheet.
In either cases, the conventional apparatuses use the clutches or solenoids in order to ensure the rotation of the pick-up roller and the separation of the pick-up roller from the sheet, and, thus, have the following problems:
(1) the control is complicated, PA0 (2) the capacity of the power source must be increased, and PA0 (3) the construction or structure is complicated.
The U.S. Pat. No. 4,262,894 and the Japanese Patent Laid-Open No. 60-204566 disclose a sheet feeding apparatus wherein a pick-up roller is attached to a pivotable arm and the pivotable arm is pivoted or rocked by a motor through a friction clutch so that the arm is shifted toward and away from the sheet. In these conventional sheet feeding apparatuses, the friction clutch is so designed that a required torque can be transmitted to the pick-up roller when the pick-up roller is lifted (i.e., separated from the sheet). In this case, however, when the pick-up roller is lowered (i.e., engaged with the sheet), nevertheless a torque less than that required in the lifting of the pick-up roller is required, the friction clutch is slipped by a more excessive force than the required one after it has contacted the sheet, and, thus, the driving force is consumed wastefully. Particularly, when a single motor is used for supplying the driving force not only to lift and lower the pick-up roller, but also to rotate the pick-up roller and other rollers, the consumption of such wasteful driving force reduces the supply of the driving power to other rollers and also reduces rotational speeds of the rollers, thus resulting in the poor or erroneous feeding of the sheet. In order to avoid this, if a large motor is used, the overall dimension of the sheet feeding apparatus itself will be large-sized and the consumption of electric power will also be greatly increased.